I know what its worth because I know my market, I also look for deed transfer in the neiborhood, its public in my state. You could just ask the selling agent or an agent friend, or pay a random agent to give you all deed transfer in the last 6months in X area. You could also put a "clause" (no clue what the english word is) to visit the house a 2nd time, to the entire satisfaction of the buyer and then hire a professional appraiser to visit with you that day and confirm the actual FMV. Obviously you dont tell the seller you will hire an appraiser, you just tell him you want another visit.You can back out of buying if you were wrong about the value with good wording on the contract. I find deals the same way an agent finds properties to put under contact, I knock on doors and pick up the phone, I dont check the MLS much anymore, I should start again.

Btw a lot of investors consider principal payment a bonus, its not really a factor. Its a great selling point to tell someone with little knowledge, thats about it. When you look at cashflow it doesnt matter if you pay 4% interest or 50%, you know the building is profitable.

The opportunity cost of debt serviced is nill because he's liquidating anything that accumulates, which is basically how i'm suggesting to compute it - to strip the equity contribution out of the cash flow.